Humans are fairly familiar with the idea of dads providing care to young children. However, paternal care is actually rare across vertebrate species. Instead, infanticidal behaviours, where dads show aggression towards young, are common. In rodents like the Mongolian gerbil, dads switch between slaughtering the youngsters of other parents and caring for their new-borns. Without this switch, dads would hurt their own young. Ana Martínez and colleagues at the National Autonomous University of Mexico used a combination of behavioural and physiological studies to investigate how the rodents make the switch from assailants to great fathers.

First, the researchers determined whether social factors trigger the gerbil's transition from aggressive attacker to paternal carer. Martínez and colleagues chose males that were aggressive towards pups and paired them with females to mate. The team then tested the males’ reactions to pups over the course of the females’ pregnancies and after the birth of their young.

After mating, the gerbils were great dads, even though the pups that were given to them were not their own offspring. This suggests that sex is one social factor that triggers paternal behaviours. Unexpectedly, however, the effects of sex were short lived: males living with pregnant partners bit any pups introduced into their cages. Fortunately, the males transformed into nurturing fathers after the birth of their own young and their paternal behaviours continued well after the delivery, despite the fathers not having sex. This suggests that there is probably another social factor besides intercourse, such as the birth, which induces paternal behaviours.

Next, the researchers looked to hormones associated with parenting, such as oestrogens and androgens, to understand the mechanism regulating the switch to caring papa. They measured circulating blood testosterone levels and quantified the density of both oestrogen and androgen receptors in three brain areas commonly associated with maternal behaviours in rodents: the medial preoptic area, the medial amygdala and the olfactory bulb.

They discovered that the testosterone levels were higher in all of the males that were gentle with pups versus males that bit the little ones. This was not surprising as injection of aggressive males with testosterone in a previous experiment had caused the aggressive behaviour to stop. This suggests that increases in circulating testosterone play a role in initiating the switch from infanticidal to paternal behaviours.

Additionally, Martínez and her colleagues observed that the densities of the oestrogen and androgen receptors were higher in every brain area examined in the gentle fathers, suggesting that oestrogenic and androgenic pathways might play a role in regulating the caring switch. The increased density of androgen receptors in the new parents that were great dads, but had not had sex recently, further supports the idea that androgen receptors in these brain areas are directly associated with paternal behaviours and not just copulation.

So, social factors, like intercourse, might increase testosterone levels in the blood to initiate male parental care. While previous studies have shown the effects of testosterone on male parenting, this is the first study to show that androgen and oestrogen receptor densities in the brain are connected to the transition from murderer to doting father. It seems that sex makes for better dads and the androgenic and oestrogenic pathways in the brain hold the key.

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